Commercially available devices used to generate hot water or to heat water miscible liquids generally fall into two categories: indirect heaters and direct heaters.
Indirect heaters accomplish heat transfer by conduction through a metal wall, such as by the direct-fired heating of liquid in a storage tank with open flames or electrical heat coils, or as in heat exchangers wherein liquid is heated by pumping through coils where coils are surrounded by steam or other heating fluid as in typical shell-and-tube heat exchangers.
Direct contact heaters accomplish heat transfer by injecting steam directly into the liquid to be heated, such as by: sparging steam into a tank containing liquid to be heated; mixing tee contact of steam and liquid to be heated; using a venturi steam injector; or by Pick-type heaters. Direct steam injection is recommended for facilities having process steam where small, medium and large volumes of heated process liquid (or hot water) are required, and where accurate temperature control is needed. Indirect heating systems are not efficient in these applications.
The advantages of direct steam injection to heat a liquid include: energy efficiency, i.e. the entire heating value of the steam is used to heat liquid (both latent and sensible heat); provides a continuous supply of heated liquid; provides accurate temperature control; and direct heaters are compact.
Heating a liquid in a vented storage tank by bubbling steam through the liquid has been used for many years because it is a simple and inexpensive way to heat liquids; however, sparging steam into a tank has disadvantages. These disadvantages include: the difficulty to arrive and hold liquid at a set temperature; energy inefficiency, because steam bubbles break to the surface and flash to the atmosphere; and severe water hammer because of large steam bubbles.
Mixing tees, as the name implies, heat a liquid by blending steam with cold liquid flowing in a pipe "tee". This equipment is simple and inexpensive, but is limited to small sizes, less than 2 inches in diameter. In many circumstances, it is impossible to achieve more than an approximate temperature. Large steam-to-liquid volumes produce severe water hammer because of the presence of large steam bubbles resulting from poor mixing.
Venturi injectors heat liquid by injecting steam into cold liquid flowing through a venturi-shaped conduit or constriction in the liquid line. The steam is injected at high velocity at the venturi throat. The low pressure zone created in the downstream expanding section of the venturi generates turbulence and promotes rapid mixing of steam bubbles with cold liquid. Venturi injectors work well when operating close to their design liquid and steam flow rates. With liquid flow rate turndown, the mixing intensity downstream of the venturi throat quickly looses intensity and is not capable of mixing and condensing the steam. Piping hammer also becomes a severe problem. Venturi injectors are not recommended for variable liquid flow rates.
Pick heaters are used in applications demanding small, medium and large volumes of heated liquid and the ability to greatly increase/decrease liquid flow rate at a fixed outlet temperature and/or increase/decrease outlet temperature at a fixed liquid flow rate. Pick heaters exhibit excellent steam turndown (outlet liquid temperature) at design liquid flow rate, and, with an external pump recirculation loop, good liquid turndown at a constant outlet liquid temperature.
The Pick heater consists of a pipe housing with helical grooves protruding inside the pipe, and running along the center axis, a perforated steam injection tube. Steam sparges out of the perforated tube through small holes along the length of the injector tube. The design works well because the steam is introduced into the liquid over a long length through many small holes, and in addition, the steam bubbles and liquid are mixed by the helical grooved wall of the housing.
With decreasing steam requirements, the spring loaded piston in the perforated steam injector tube lowers (controller decreases steam pressure). This sparges the lesser amount of required steam through a smaller number of holes and forces steam out near the front of the helical mixer section. Small steam bubble size is assured by sparging less steam through a smaller number of holes. A major disadvantage of the moving piston design is that piston is prone to sticking due to tight piston/perforated injector tube tolerences. When piston sticks at the low steam flow position, and a high steam rate is called for, high steam velocities can cause extensive damage. Control problems and maintenance downtime results if piston sticks in any position.
While the Pick heater is an effective direct contact heater, it is desirable to provide an improved type direct contact heater and method of heating which requires less maintenance, is of low cost, and can be constructed of a wide variety of materials.